Oil sands have become an attractive source of oil recovery to support global demand for oil. Oil sands are large deposits of naturally occurring mixtures of bitumen, water, sand, clays, and other inorganic materials found on the earth's surface. Bitumen is a highly viscous form of crude oil. The largest oil sands deposits are found in Canada and Venezuela. In particular, the Athabasca oil sands deposit is equivalent to 1.6 to 2.7 trillion barrels of oil, and is located in the Canadian provinces of Alberta and Saskatchewan. About 10% of the Athabasca oil sands deposit can be mined. Once the oil sands are mined, it is processed by extracting the bitumen.
The bitumen must be extracted and separated from the water, sand and fine clays of the oil sands. Today, the oil sands are mined, crushed, then mixed with hot water, and optionally chemicals, to facilitate extracting the bitumen from the sand and clay fines. The extracted bitumen is separated from the sands and fine clays and is then refined. The remaining sand, clays and water, commonly referred to as “tailings”, are further processed to dewater the sand and clays. The sand and clay are typically disposed, e.g., in a tailings pond and settle to become mature fine tailings. Mature fine tailings are a stable slurry comprising fine clays and sands, silt, water, and bitumen. Mature fine tailings have no strength, no vegetative potential and may be toxic to animal life, so must be confined and prevented from contaminating water supplies. The recovered water from the dewatering step may be re-used in the extraction process. Faster recovery of the water reduces heat energy requirements when this water is recycled for use in the extraction process.
The recovered bitumen from this process is in the form of a froth. The froth comprises a concentrated bitumen (typically 50% or greater), water, fine clays and sands. The froth is treated in a froth treatment unit, which may use steam (to de-aerate the froth) and a naphthenic or paraffinic solvent to recover a bitumen with greater than 95% purity. A byproduct of the froth treatment process is a froth treatment tailings. The froth treatment tailings comprise water, residual solvent, and fine solids that are primarily smaller than 44 micrometers in size. The froth treatment tailings are typically disposed of in a tailings pond. Froth treatment tailings contribute to mature fine tailings formation.
Tipman et al., in U.S. Pat. No. 5,876,592, disclose recovery of bitumen from oil sands in a process comprising adding aqueous caustic to an oil sands slurry, to create an emulsion. The emulsion is allowed to separate into 3 layers, with a top layer of a first froth comprising bitumen, bottom layer, referred to as tailings, comprising water, sand and clay fines that settled, and a middle layer, referred to as middlings, comprising residual bitumen, suspended clay fines and water. The middlings are further processed to recover additional bitumen in the same manner as the oil sands slurry, producing a second froth. The second froth may be combined with the first froth to recover bitumen by dilution with a solvent and removal of sand and clay fines.
Yuan, et al., Canadian Metallurgical Quarterly, 2007, vol. 46, no. 3 pp. 265-272, disclose using a multiple-step process, in a particular sequence, for removing sands and fine clays from tailings. The first step is referred to as flocculation-coagulation-flocculation (FCF), in which a flocculant is added. This allows for the flocculation of larger particles leaving fines in solution. In the second step, a coagulant is added. The coagulant destabilizes the fines causing small flocs to form. In the third step, a small amount of flocculant is added to combine the larger flocs from the first step with the smaller flocs in the second step, resulting in even larger flocs and an increase of settling rates, allowing for faster dewatering.
Acidified silicate has been used to enhance bitumen extraction by Masliyah, Ind. Eng. Chem. Res., 2005, vol. 44, pp. 4753-4761. By acidifying the silicates, divalent metal ions can be sequestered allowing for improved bitumen liberation while maintaining consistent pH. There is a similar disadvantage with this process as found in WO 2005/028592, that is, solids are dispersed.
Li, Energy & Fuels, 2005, vol. 19, pp. 936-943 disclose the effect of a hydrolyzed polyacrylamide (HPAM) on bitumen extraction and tailings treatment of oil sands ores. Careful control of HPAM dosage is necessary to achieve efficiency in both bitumen extraction and in flocculation of solid fines.
Chaiko et al., in U.S. Pat. No. 6,153,103, disclose a method to separate and recover ultra fine particles and soluble salts from a dilute process streams using sodium silicates and organic gelling agents through syneresis process. This method is used for dilute solutions and for solids to silicate ratios of 0.4:1 or less.
Separation of bitumen from sand and clay fines, as well as dewatering of the sand and clay fines for disposal, are especially difficult for so-called “poor quality ores.” Generally, a poor quality ore, in reference to an oil sands ore is an oil sands ore that contains a large amount of fines that hinder, not only extraction of bitumen, but also the dewatering process of sand and clay fines. Poor quality ores are difficult to extract bitumen from at acceptable yields using conventional methods. In addition, more bitumen is retained in the tailings streams from extraction of poor quality ores, which is sent to the tailings pond as a yield loss.
Poor quality ores reduce yield by as much as 35 to 50% and are avoided when possible. Alternatively, poor quality ores are blended in limited quantities with good quality ores so they can be processed more effectively. With demand for oil increasing every year, there is a need to mine these poor quality ores and to produce high yield of bitumen. The tailings should be essentially free of bitumen and separated from water, so the water can be re-used and the solids can be returned to the environment free of bitumen, within environmental limits.
There is a desire to have lower extraction temperatures (for example, less than about 50° C.) to save heat energy. For example, when an adjacent upgrading facility to treat the extracted bitumen is not available, there is added cost to supply heat energy for the extraction water.
Mature fine tailings ponds also pose an environmental concern. Often disposal of the tailings creates ponds where the clays and fines remain suspended in water and ultimately become mature fines tailings. The Energy Resources Conservation Board of Canada has issued Directive 074, which mandates a reduction of fine tailings ponds and the formation of trafficable deposits for all oil sands operators. Currently, these mature fine tailings are treating with gypsum/lime and centrifuging. Gypsum/lime treatment is undesirable due to the added calcium ions in and around the tailings pond and the remaining solids are too soft to be trafficable for long periods of time. Centrifuging is undesirable due to the large capital investment and having to transport the mature fine tailings to centrifuge locations.
While there have been many advances in the oil sands extraction and tailings, there remains a need to improve bitumen recovery (yield) from oil sands, improve de-watering of the tailings (i.e., less water in the tailings) and reduce need to add fresh water bitumen recovery processes. There is also a need to improve bitumen extraction in poor quality ores, and to do so without significant capital equipment, without significant bitumen yield loss. There is also a need to reduce or eliminate mature fine tailings ponds where the remaining solid can be useful. The present invention meets these needs.